Ok this might sound a little funny, but is it ok to use ro di water for a betta? I have kept saltwater tanks for a few years now and I was just wondering if they would be fine with this water. I read that they don't do well in distilled water and just wondered if they are good in rodi.

No, it wouldn't be a good idea. Pure Ro di water has pretty much no buffering capacity, so you would be having major problems with the pH level. There are also some trace elements that the Betta will actually need that won't be present in that type of water. And, if you have live plants, they will die in this type of water.
Possibly, you could use Ro di water and then add Seachem equilbrium to help solve those problems.

Just use normal tap water and use Conditioner on it, it is so much more healthy for Bettas and the plants. You can always add half a teaspoon per gallon of Aquarium Salt which will help your betta(less for plants in the tank).